Fibrous tape

ABSTRACT

The invention relates to a fibrous tape made from fibers comprising highly oriented polymer, the tape having a tenacity of at least 1.2 N/tex and an areal density of between 5 and 250 g/m 2 , wherein the tape has a transversal strength of at least 0.5 MPa. The invention also relates to sheets comprising the tape of the invention and antiballistic articles comprising at least two of said sheets. The invention further relates to a process for the preparation of the tapes of the invention.

The invention relates to a fibrous tape made from fibers comprisinghighly oriented polymer, the tape having a tenacity of at least 1.2N/tex and an areal density of between 5 and 250 g/m². The inventionfurther relates to a process to manufacture said fibrous tape from saidfibers comprising highly oriented polymer.

Such tape is known from WO2013/131996. WO2013/131996 discloses a fibroustape having a tenacity of at 3.54 N/tex and an areal density of about 35g/m² made from a plurality of fused high tenacity UHMWPE filaments. Thetape disclosed in WO2013/131996 is further in contact with a plastomerlayer at an areal density of between 0.2 and 15 g/m².

Although the tapes according to WO2013/131996 show satisfactory strengthand performance in applications such as antiballistic panels, they showa deficiency during handling of the tapes and/or the sheets comprisingthe tapes which is expressed by the occurrence of defects in the tapedue to unwanted splitting of the tape. Furthermore, the performance ofthe tape in ballistic applications can be further improved.

The object of the present invention is to provide a fibrous tape withoptimized handling properties resulting in less defects of the tapethrough splitting. A further objective of the invention may be toprovide a tape with improved antiballistic performance.

This objective is achieved according to the invention by providing atape with a transversal strength of at least 0.5 MPa. It was observedthat tapes with such improved transversal strength may provide improvedhandling properties. It appeared that tapes according to the inventioncan be handled more easily and may be processed into antiballisticsheets and antiballistic panels with substantially less defects. It wasfurther observed, that the tapes according to the invention may provideantiballistic sheets and panels with optimized antiballisticperformance.

Fibrous tapes are also known from other patent applications such asWO2012/080274 and WO2013/130160. Also the therein disclosed fibroustapes have high tenacity but also present the above describeddeficiencies.

By the term “fibrous tape” is herein understood a tape obtained by aprocess wherein fibers comprising polymer are used as a precursormaterial. A fibrous tape is structurally different from a non-fibroustape, which is usually obtained by compressing polymeric powders orspinning solutions or melts of polymers. The cross-section of a fibroustape according to the invention, if observed with a microscope,possesses boundaries between the fibers forming the tape. The observableboundaries of the precursor fibers may be recognized as substantiallystraight limits between the precursor fibers in the fibrous tape, whichprecursor fibers may possess a mainly polygonal cross-section, forexamples a hexagonal, pentagonal or rectangular cross-section.

The fibrous tape comprises abutting polymeric fibers having a fiberlength, wherein the abutting fibers may be fused to each other over anabutting length. Preferably a plurality of fibers, i.e. more than onefiber, is used to make such tape, the plurality of fibers may beprovided by a single or more than one yarn comprising the fibers.Preferably, the abutting length is at least 50% of the fibers' length,more preferably at least 70%, most preferably at least 90%. Morepreferably, the abutting length of the polymeric fibers is about thesame with the fibers' length. The abutting length over which abuttingpolymeric fibers may be fused to each other is a measure of the degreeof fibers' fusion. The degree of fibers' fusion may be adjusted as itwill be detailed hereinafter and the abutting length may be measuredwith a microscope preferably provided with an adjustable depth of fieldand/or with a contrast enhancer device. The difference between two, atleast partially, fused fibers and two non-fused fibers is that the fusedfibers are hindered in moving one in respect to each other over thefused part which keeps the fibers in contact. Accordingly a fibrous tapein the context of the present invention is structurally different fromthe monolayers known in the art comprising fibers and an elastic resinor a polymeric matrix that encapsulate and holds the fibers together. Incontrast to said monolayers known in the art, the fibers of the presenttapes are essentially held together by the above described interactionof abutting fibers. The present fibrous tapes are substantially devoidof resins or adhesives located in between the fibers forming the tape.Preferably the fibrous tapes are substantially devoid of resins oradhesives. By substantially devoid is understood that the fibrous tapescomprise less than 5 wt %, preferably less than 3 wt %, more preferablyless than 2 wt % and most preferably less than 1 wt % of resin oradhesive.

By tape is herein understood an elongated body having a longitudinaldirection, a width, a thickness and a cross-sectional aspect ratio, i.e.the ratio of thickness to width. Said cross-section is defined assubstantially perpendicular to the longitudinal direction of the tape.The longitudinal direction or machine direction of the tape essentiallycorresponds to the orientation of the fused fibers. The length dimensionof a tape of the invention is not particularly limited. The length mayexceed 10 km and mainly depends on the polymeric fibres and the processused to produce the tape. Nevertheless said tape can for conveniencereasons be manufactured to smaller sizes, according to the requirementsof the envisioned applications.

In a preferred embodiment, the tape of the invention has an averagecross-sectional aspect ratio (thickness:width) of at most 1:50,preferably at most 1:100, more preferably at most 1:500, even morepreferably at most 1:1000. The width of the fibrous tape is preferablybetween 2 mm and 3000 mm, more preferable between 10 mm and 2500 mm,even more preferably between 20 mm and 2000 mm, yet even more preferablybetween 50 mm and 1800 mm and most preferably between 80 mm and 1600 mm.The fibrous tape preferably has a thickness of between 1 μm and 200 μm,more preferably of between 3 μm and 120 μm, even more preferably ofbetween 5 μm and 100 μm, even more preferably of between 8 μm and 80 μmand most preferably of between 10 μm and 50 μm. By width is hereinunderstood the largest dimension between two points on the perimeter ofa cross-section of the tape, said cross-section being orthogonal to thelength of the tape. By thickness is herein understood a distance betweentwo points on the perimeter of said cross-section, said distance beingperpendicular on the width of the tape. The width and the thickness of atape can be measured according to known methods in the art, e.g. withthe help of a ruler and a microscope or a micrometer, respectively. Itwas observed that in contrast to the tapes of the prior art, the tapesaccording to the invention can be produced within above preferred widthsand thicknesses while a low amount of defects of the tapes onceprocessed into antiballistic articles is maintained.

By fiber is herein understood an elongated body having a length muchgreater than its transverse dimensions. A fiber may have a regularrounded cross-section, e.g. oval or circular; or an irregularcross-section, e.g. lobed, C-shaped or U-shaped. The fibers may havecontinuous lengths, known in the art as filaments, or discontinuouslengths, known in the art as staple fibers. Staple fibers are commonlyobtained by cutting or stretch-breaking filaments. A yarn for thepurpose of the invention is an elongated body containing many fibers.The fiber has a cross sectional aspect ratio, i.e. the ratio of thelargest dimension between two points on the perimeter of a cross-sectionof the fiber to the lowest dimension between two points on the sameperimeter. Preferably the cross-sectional aspect ratio of the fiber isat most 10:1, more preferably of at most 5:1 and even more preferably3:1.

Examples of fibers of polymer suitable for the present invention includebut are not limited to fibers manufactured from polyamides andpolyaramides, e.g. poly(p-phenyleneterephthalamide) (known as Kevlar®);poly(tetrafluoroethylene) (PTFE);poly{2,6-diimidazo-[4,5b-4′,5′e]pyridinylene-1,4(2,5-dihydroxy)phenylene}(known as M5); poly(p-phenylene-2, 6-benzobisoxazole) (PBO) (known asZylon®); poly(hexamethyleneadipamide) (known as nylon 6,6),poly(4-aminobutyric acid) (known as nylon 6); polyesters, e.g.poly(ethylene terephthalate), poly(butyleneterephthalate), and poly(1,4cyclohexylidenedimethyleneterephthalate); polyvinyl alcohols;thermotropic liquid crystal polymers (LCP) as known from e.g. U.S. Pat.No. 4,384,016; polyolefins e.g. homopolymers and copolymers ofpolyethylene and/or polypropylene; and combinations thereof.

Good results may be obtained when the polymer is a polyolefin,preferably a polyethylene. Preferred polyethylenes are high or ultrahighmolecular weight polyethylene (UHMWPE). Polyethylene fibers may bemanufactured by any technique known in the art, preferably by a melt ora gel spinning process. Most preferred fibers are gel spun UHMWPEfibers, e.g. those sold by DSM Dyneema, NL trademarked as Dyneema®. If amelt spinning process is used, the polyethylene starting material usedfor manufacturing thereof is preferably a high molecular weightpolyethylene with a weight-average molecular weight between 20,000 and600,000 g/mol, more preferably between 60,000 and 200,000 g/mol. Anexample of a melt spinning process is disclosed in EP 1,350,868incorporated herein by reference. If the gel spinning process is used tomanufacture said fibers, preferably an UHMWPE is used with an intrinsicviscosity (IV) of preferably at least 5 dL/g, more preferably at least 8dL/g, most preferably at least 12 dL/g. Preferably the IV is at most 40dL/g, more preferably at most 30 dL/g, more preferably at most 25 dL/g.Preferably, the UHMWPE has less than 1 side chain per 100 C atoms, morepreferably less than 1 side chain per 300 C atoms. Preferably the UHMWPEfibers are manufactured according to a gel spinning process as describedin numerous publications, including U.S. Pat. No. 4,413,110, GB 2042414A, GB-A-2051667, WO 01/73173 A1.

The tenacity or tensile strength of the polymeric fibers is preferablyat least 1.2 N/tex, more preferably at least 2.5 N/tex, most preferablyat least 3.5 N/tex. Best results were obtained when the fibers ofpolymer were UHMWPE fibers having a tenacity of at least 2 N/tex, morepreferably at least 3 N/tex.

The tenacity of the tape of the invention is preferably at least 1.5N/tex, preferably at least 2.0 N/tex, more preferably at least 2.5N/tex, even more preferably at least 3.0 N/tex and most preferably atleast 3.5 N/tex. It was observed that tapes with increased tenacity,sheets and panels with further improved ballistic properties can beobtained.

The fibrous tape of the invention has a transversal strength of at least0.5 MPa. Achieving such transversal strength came as a surprise for theinventors as it is known in the art that increased transversal strengthof tapes usually come to the expense of other mechanical properties suchas tenacity. It is considered to be an achievement of the inventors tohave identified a process allowing for the first time to produce fibroustapes with transversal strength exceeding 0.5 MPa while substantiallymaintaining the tenacities of the employed fibers. Preferably thetransversal strength of the tape of the invention is at least 0.6 MPa,more preferably at least 0.7 MPa, even more preferably at least 0.8 MPaand most preferably at least 0.9 MPa. It was observed that increasingthe transversal strength to such preferred levels further improved thehandling properties of the tapes and may further reduced the amount ofdefects in the antiballistic articles made thereof. By transversalstrength of a fibrous tape in the context of the present invention ismeant the force, in Newton (N), required to rupture a tape along across-sectional area perpendicular to its width direction divided by thesurface (in mm²) of said cross-sectional area. Said transversal strengthis thus expressed in MPa or alternatively in N/mm². Further details asto the measurement of the transversal strength can be found in theMethods of Measuring.

It has further been found that the balance between transversal strengthand tenacity could be further improved if the fibers from which the tapeaccording to the invention are produced contain between 10 ppm and 1 wt% of a solvent for the polymer from which the fibers are made, whereinthe weight percentage is expressed as weight solvent per total weight ofthe fiber. Accordingly a preferred embodiment of the tapes of thepresent invention is that the fibers comprising polymer from which thetapes have been made comprise at least 10 ppm, preferably 20 ppm mostpreferably 50 ppm of a solvent for the polymer. Contents higher than 1wt % no longer essentially contribute to the improvement, or even impairthe transversal strength. For the above reasons, the solvent content inthe fibre is preferably from 10 ppm to 1 wt %, more preferably 20 ppm to0.5 wt %, even more preferably between 50 ppm to 0.1 wt %, and mostpreferably between 0.01 wt % to 0.1 wt %.

Solvent is here understood to be a substance that is capable ofdissolving the polymer in question. Suitable solvents for polymers areknown to one skilled in the art. They can, for example, be chosen fromthe ‘Polymer Handbook’ by J. Brandrup and E. H. Immergut, third edition,chapter VII, pages 379-402. Examples of suitable solvents forpolyolefins, in particular for polyethylene, are, separately or incombination: decalin, tetralin, toluene, lower n-alkanes such as hexane,(para-)xylene, paraffin oil, squalane, mineral oil, paraffin wax,cyclooctane. For the reasons cited above, the solvent is most preferablyparaffin oil, paraffin wax or decalin.

Preferably, the solvent is a high boiling solvent, such as paraffin oil.It was observed that such solvents provide fibrous tapes with furtherimproved transversal strength. Preferably, these are solvents having aboiling temperature that is substantially higher, preferably at least 50K, more preferably at least 100 K higher, than the melting temperatureof the polymer. The melting temperature of the fibers can be determinedby DSC using a methodology as described at pg. 13 of WO 2009/056286.

The presence of the solvent in the fiber may have multiple origins. Forexample the solvent present in the fibre may be a remainder of solventused during the spinning process of the fibre or it may have beenpurposely added before, during or after the spinning process of thefibre or the manufacturing of the fibrous tape.

In the context of the present invention fibres of highly orientedpolymer is defined as that the polymer chains run substantially parallelwith the direction of fiber. It is preferred for the degree oforientation F to be at least 0.95, more preferably at least 0.97 andeven more preferably at least 0.98. The degree of orientation is definedby the formula F=(90°−H°/2)90°, where H° is the width at half the heightof the scattering intensity along the Debye ring of the strongestreflection on the equator.

The fibrous tape, or the therefrom produced sheets and/or theantiballistic articles of the invention may also comprise a binder or amatrix material. Said binder or matrix material may be present inbetween the polymeric fibers or in between the fibrous tapes. Variousbinders or matrices may be used, examples thereof includingthermosetting and thermoplastic materials. A wide variety ofthermosetting materials are available, however, epoxy resins orpolyester resins are most common. Suitable thermosetting andthermoplastic materials are enumerated in, for example, WO 91/12136 A1(pages 15-21) included herein by reference. From the group ofthermosetting materials, vinyl esters, unsaturated polyesters, epoxidesor phenol resins are preferred. From the group of thermoplasticmaterials, polyurethanes, polyvinyls, polyacrylics,polybutyleneterephthalate (PBT), polyolefins or thermoplasticelastomeric block copolymers such aspolyisopropene-polyethylene-butylene-polystyrene orpolystyrene-polyisoprene-polystyrene block copolymers are preferred.

More preferred, however, is that the fibrous tape is substantially freeof any binder or matrix material between the polymeric fibers. It wasobserved that in the absence of binders or matrix materials, theballistic properties of the material of the invention may be improved.

Yet in a preferred embodiment, the binder or matrix material is presenton and in between the fibrous tapes as for example disclosed inWO2013/131996, especially on pages 9, 11 and 12, included herein byreference.

The invention further relates to a process for the manufacturing of thetapes of the invention, comprising the steps of:

-   -   (a) providing fibers comprising highly a oriented polymer, said        fibers having a tenacity of at least 1.2 N/tex;    -   (b) forming a layer comprising the fibers;    -   (c) applying a longitudinal tensile force to the fibers in the        layer,    -   (d) stretching the fiber layer at a draw ratio of at least 1.01        to form a stretched layer;    -   (e) providing the stretched layer at a processing temperature        T_(p) to compression means;    -   (f) compressing the stretched layer of fibers by subjecting the        layer to a compression by the compression means, the compression        means having a temperature T_(c), to form a fibrous tape;    -   (g) optionally stretching the fibrous tape by a draw rate of at        most 1.1 and,    -   (h) cooling the fibrous tape to a temperature of at most 80° C.        under a tension sufficient to prevent loss of mechanical        properties;        wherein T_(m) is the melting temperature of the polymer, wherein        T_(m)>T_(p)≧T_(m)−30 K, and wherein T_(c)≦T_(p)−3 K.

It was observed that with the process of the invention, a tape havingincreased transversal strength as compared with known fibrous tapes maybe obtained.

It was further observed that the mechanical properties of the fibroustape made according to the process according to the invention aresimilar to the mechanical properties of the fibers utilized tomanufacture the tape thereof. This came also as a surprise sincehitherto the mechanical properties of tapes manufactured from polymericfibers were usually much lower than those of the polymeric fibers.Accordingly the present invention also relates to a fibrous tapeobtainable by the process of the invention. Preferably the fibrous tapeobtainable by the process of the invention has a tenacity which is atmost 20% lower than the tenacity of the polymeric fibers used tomanufacture said fibrous tape, more preferably at most 10%, mostpreferably with at most 5% lower tenacity than the polymeric fibers usedto manufacture the fibrous tape. If polymeric fibers with varioustenacities and moduli are used to manufacture the tape of the invention,the tenacity or modulus of the polymeric fibers to be considered are anaverage tenacity and modulus of the various polymeric fibers.

Preferably, at step (a) of the process of the invention, the pluralityof highly oriented polymer fibers is provided as at least one yarn, morepreferably more than one yarn, that may be twisted or untwisted.Preferably the yarns have a twist of less than 1 per 100 cm yarn, morepreferably less than 1 twist per 200 cm yarn and even more preferablyless than 1 twist per 400 cm. Most preferably the yarns aresubstantially untwisted. In case a twisted yarn is provided to theprocess, the skilled person will be aware of means to remove the twistfrom the provided yarns before or during the formation into a layercomprising the fibers, step (b).

According to the process of the invention, at step (b) the polymericfibers are formed into a layer comprising the fibers, preferably a layerof fibers. Said layer may be fibers arranged in configurations ofvarious types which may comprise random or ordered oriented fibers suchas arranged in parallel arrays. Most preferred the layer of fibers is anunidirectional network wherein a majority of fibers, e.g. at least 50mass %, more preferably at least 75 mass %, even more preferably atleast 95 mass %, most preferably about 100 mass % of the total mass offibers forming the layer, is arranged to run substantially in parallelalong a common direction. The unidirectional alignment of polymericfibers may be achieved through various standard techniques known in theart that are able to produce substantially straight rows ofunidirectionally aligned fibers, such that adjacent fibers overlap andpreferably there is substantially no gap between them. An example ofsuch a technique is described in WO 2009/0056286 included herein byreference, wherein a layer comprising abutting and unidirectionallyaligned polymeric fibers may suitably be formed by feeding a polymerfiber from an unwinding station under tension, through an alignmentmeans, e.g. a reel followed by a plurality of spreader bars. It wasobserved that such substantial parallel alignment of the fibers in thelayer provides tapes with further improved transversal strength.

The thickness of the layer comprising the polymeric fibers is preferablychosen to yield after the stretching of steps (d) and compression step(f) the desired thickness of the tape. The layer may have a minimumthickness of about the diameter of the fibers. Preferably the thicknessof the layer will be at least twice the thickness of the fibers.

Preferably, the process of the invention comprises an additional step(b1) wherein the fibers are preheated to a temperature below T_(m),before or while stretching the layer in step (d). Preheating of thelayer may be carried out by keeping the layer for a dwell time in anoven set at a preheating temperature, subjecting the layer to heatradiation or contacting the layer with a heating medium such as aheating fluid or a heated surface. Preferably, the preheatingtemperature is between T_(m)−2 K and T_(m)−30 K, more preferably betweenT_(m)−3 K and T_(m)−20 K, most preferably between T_(m)−5 K and T_(m)−15K. The dwell time is preferably between 2 and 100 seconds, morepreferably between 3 and 60 seconds, most preferably between 4 and 30seconds.

During the process of the invention, in step (d) the layer is stretchedat a draw ratio of at least 1.01. More preferably the draw ratio is atleast 1.03, even more preferably at least 1.05 and most preferably atleast 1.08. The maximum draw rate that may be applied on the layer mayessentially be limited by the drawability of the fibers employed in theprocess. Nevertheless it was observed that too high draw rates appliedto the layer before the compaction step (f) may result in unwanteddeficiencies of the produced tapes, such as fibrillation or splitting ofthe tapes during or after the processing of the fibrous tape.Accordingly, the stretching in step (d) is preferably limited to a drawratio of less than 2.0, preferably less than 1.8, more preferably lessthan 1.5 and most preferred less than 1.3. In a yet preferred embodimentthe draw ratio of the layer may be between 1.01 and 2.0, preferablybetween 1.03 and 1.8, more preferably between 1.05 and 1.5 and mostpreferably between 1.08 and 1.3. It was observed that at such limiteddraw ratio fibrous tapes having further improved properties may beobtained.

In an alternative process of the invention, the process may form anintegral part of the manufacturing process of high tenacity fibers andwherein the stretching step (b) is the last stretching step of thedrawing operation to which the fibers are subjected. Depending on thenumber of drawing steps and the respective drawing ratios in saiddrawing steps the draw ratio of step (d) may be between 2.0 and 10,preferably between 2.5 and 9.0, more preferably between 3.0 and 8.0, andmost preferably between 4.0 and 7.0. It was observed that combination ofthe drawing step (d) with the production of high tenacity fibers has asubstantial efficiency advantage while the transversal strength of theproduced tape remains substantially unaffected.

In step (e) the layer is provided at a temperature T_(p) to compressionmeans. The temperature T_(p) may be achieved by heating or cooling thelayer with means known to the skilled person. Preferably the temperatureT_(p) is between T_(m)−1 K and T_(m)−30 K, more preferably betweenT_(m)−1 K and T_(m)−15 K, most preferably between T_(m)−1 K and T_(m)−5K.

At step (f) of the process of the invention, the layer comprising thepolymeric fibers is compressed by the compression means. Preferably thecompression means may be a calender, a smoothing unit, a double beltpress, an alternating press. The compression means form a gap throughwhich the layer will be processed. Preferably, said layer is introducedinto said gap with an inline speed of at least 1 m/min, more preferablyof at least 2 m/min, most preferably of at least 3 m/min. Thetransversal pressure to which the layer is subjected may be expressed inN/mm or N/mm² depending on the geometry of the compression means. In thecase the compression means is a calender or a comparable compressionmeans applying a compression to a narrow surface area, the line pressureis at least 100 N/mm, more preferably at least 200 N/mm, even morepreferably at least 300 N/mm, most preferably at least 500 N/mm. In casethe compression means is a press, i.e. applying a compression to a broadsurface, the surface pressure is at least 1 N/mm², more preferably atleast 5 N/mm², even more preferably at least 10 N/mm², most preferablyat least 20 N/mm². It was observed that the higher the respectivepressure, the higher the transversal strength of the fibrous tapes is.It is commonly known in the art that a calender comprises at least twocounter-rotating calendering rolls which form a nip, e.g. where therolls abut each other, by applying a, preferably constant, closing forceon said rolls. The closing force is usually measured by a force gauge.The calendering line pressure can therefore be easily determined bydividing the closing force as measured by the force gauge to the widthof the layer comprising the network of fibers. It is further commonlyknown in the art that a press comprises at least 2 counteractingcompression surfaces by applying a, preferably constant, closing forceon said compression surfaces and hence applying a pressure in N per mm²onto the material in between the at least 2 compression surfaces.

The compression step (f) of the process is carried out with atemperature T_(c) of the compression means, wherein T_(c) is below thetemperature T_(p) at which the layer comprising the polymer fibers isfed to the compression means. Preferably the temperature of thecompression means is at least 3 K below T_(p), preferably at least 5 K,more preferably at least 10 K, more preferably at least 20 K, even morepreferably at least 30 K and most preferably at least 50 K below theT_(p) of the layer. The inventors surprisingly found out, that byapplying lower temperatures of the compression means may provide tapeswith an optimized balance of mechanical properties. It was observed thatoperating the process according to the invention may provide fibroustapes with optimized tenacity and transversal strength. The temperatureof the compression means may be set by using internally heated or cooledcompression means. Said temperature is influenced amongst others by thedimensions of the compression means (for example the diameter of thecalendering rolls), the temperature (T_(p)) at which the layer isprovided, the inline speed and optionally the temperature applied to thespace beyond the compression means, such as a forced cooling of theproduced fibrous tape exciting the compression means. By temperature ofthe compression means (T_(a)) is herein understood the temperature ofthe surface of the compression means in contact with the compressedlayer. In case said surface temperature is different for differentpositions of the compression means, the T_(c) is measured at theposition where the fibrous tape is released from the compression means.In case the compression means is a calender, the calendering rollspreferably have a diameter of between 100 mm and 1000 mm, morepreferably between 200 mm and 700 mm, most preferably between 300 mm and600 mm.

In an optional embodiment the fibrous tape is stretched in step (g) at adraw rate of at most 1.1. Preferably the draw rate of the fibrous tapeis at most 1.05, more preferably at most 1.03 and most preferably atmost 1.01. It was surprisingly observed that such a limitation of thedraw rate after the compression of the polymer fibers into a tape mayprovide a fibrous tape with further increased transversal strength.

The inventive process employs highly oriented polymer fibers. Suchfibers may be fully drawn fibers, meaning that the fibers have beendrawn as much as chosen for the manufacturing of the fiber product.Since a fiber manufacturer typically designs the product to have safetymargin, a fully drawn fiber may be drawn further, though excessivefurther drawing typically will lead to fiber failure. A preferredembodiment of the inventive process is that the layer comprising thepolymeric fibers are stretched in between the steps (a) and (f) to atotal draw ratio from 1.02 to 3.0, preferably 1.03 to 2.0, morepreferably 1.05 to 1.5 and most preferably 1.08 to 1.3. It was observedthat these preferred ranges of drawing during the manufacture of thetape will optimize the balance of tape strength and transversalstrength. While higher draw rates may result in increases of the tapetenacity, they may negatively affect the transversal strength. If toolow draw rates are applied, both transversal strength and tenacity ofthe tape may be negatively affected.

In the mentioned step (h) here above, the fibrous tape is cooled suchthat the temperature of the tape is reduced with at least 25° C.,preferably the tapes are cooled to room temperature.

Accordingly, in a preferred embodiment of the process of the inventionT_(p) and T_(c) are chosen to respect the conditions ofT_(m)>T_(p)≧T_(m)−15 K, and wherein T_(c)≦T_(p)−15 K. In a furtherpreferred embodiment T_(p) and T_(c) are chosen to respect the conditionof T_(m)>T_(p)≧T_(m)−5 K, and wherein T_(c)≦T_(p)−30 K. It was observedthat if the process of the invention is operated within the aboveboundaries, the obtained fibrous tapes have optimized balance oftenacity and transversal strength and will provide antiballisticarticles with substantially reduced number of defects.

In a preferred process according to the invention the fibers comprise asa polymer UHMWPE, preferably the UHMWPE has an IV (measured at @135° C.in decalin) of between 5 dL/g to 40 dL/g, more preferably between 8 and30 dL/g and more preferably between 10 dL/g and 25 dL/g. It was observedthat such ranges of intrinsic viscosities of UHMWPE provide furtherimproved antiballistic performance of the therefrom manufactured fibroustapes.

The process of the present invention furthermore enables tapes to bemade that were never made available before, i.e. tapes with a uniquecombination of mechanical properties, i.e. a balance between ballisticproperties and handling defects. More specifically the present inventionenables fibrous tapes with a tenacity (TS) of at least 1.2 N/tex and atransversal strength (S_(tr)) of at least 0.5 MPa. In a preferredembodiment, the tenacity and the transversal strength of the taperespect the relation of formula 1;

S _(tr)=2 MPa−a*ρ*TS  Formula (1)

wherein S_(tr) is expressed in MPa, ρ is the density of the fibers ing/mm³, TS is expressed in N/tex and the factor a is at most 6.5×10⁻³,more preferably a is at most 5.0×10⁻³, even more preferably a is at most4.0×10⁻³ and most preferably at most 3.0×10⁻³. Surprisingly such tapesyield excellent performance when used in the manufacture ofantiballistic products. Such high performance is unexpected in the fieldof antiballistic products.

The invention further relates to products such as sheets andantiballistic articles comprising the fibrous tapes of the invention. Inparticular, the invention relates to a sheet comprising at least twomonolayers comprising fibrous tapes according to the invention or atleast one layer of woven fibrous tapes according to the invention.Preferably the monolayers comprise unidirectional aligned fibrous tapes.

The sheets may also contain a binder in between the tapes forming saidsheet. The purpose of the binder may be to hold said fibrous tapes inplace in order to improve the ease of operation of the monolayers orsheets comprising thereof. Suitable binders are described in e.g. EP0191306 B1, EP 1170925 A1, EP 0683374 B1 and EP 1144740 A1. It wasobserved that good results may be obtained when the sheets or thetherefrom manufactured panel is substantially free of any binder or anyother material the purpose of which being to hold the fibrous tapestogether.

By a monolayer of unidirectional aligned fibrous tapes is hereinunderstood that a majority of the fibrous tapes in the sheet, e.g. atleast 70 mass % of the total mass of fibrous tapes in said monolayer,more preferably at least 90 mass %, most preferably about 100 mass %,run along a common direction. In a sheet comprising at least twomonolayers, the direction of the fibrous tapes in a monolayer is at anangle α to the direction of a fibrous tape in an adjacent monolayer. Ina layer of woven fibrous tapes comprising weft and warp woven tapes thedirections of orientation of the weft and the warp woven fibrous tapesare at an angle β, whereby respectively α and β are preferably between20 and 90°, more preferably between 45 and 90° and most preferablybetween 75 and 90° most preferably the angles α and β are about 90°.

In a preferred embodiment, the sheets of the invention are compacted bymechanical fusing of the fibrous tapes. Said mechanical fusing ispreferably achieved under a combination of pressure, temperature andtime which results in substantially no melt bonding. Preferably, thereis no detectable melt bonding as detected by DSC (10 K/min). Nodetectable melt bonding means that no visible endothermic effectconsistent with partially melt recrystallized fibers is detected, whenthe sample is analyzed in triplicate. It has been found that theapplication of high pressures at a temperature suitably below themelting point of the fiber results in no detectable amount of meltrecrystallized fibers being present, which is consistent with thesubstantial absence of melt bonding.

Accordingly the invention also relates to compressed sheets comprisingthe fibrous tape of the invention. It was observed that said compressedsheet will have a more homogeneous appearance if compared to compressedsheets made from fibrous tapes known in the art. As presented above, thefibrous tapes known in the art are prone to longitudinal splitting uponhandling. In sheets manufactured from said prior art fibrous tapesimperfections in the form of splits may be observed. The presence ofsplits in the tapes may result in local defects due to overlaps or gapsin the tape. Hence the invention also relates to sheets comprising thefibrous tape, wherein the split length in the tape is less than 5 m/m²of sheet, preferably less than 2 m/m² of sheet, even more preferablyless than 1 m/m² of sheet, and most preferably less than 50 cm/m² ofsheet. It was observed that such low split length in a sheet comprisingthe tape improves the antiballistic performance of articles made fromsaid sheets.

The invention also relates to antiballistic articles comprising thesheet according to the invention. Preferably the antiballistic articlecomprising at least 2, preferably at least 4, more preferably at least 8sheets. It was observed that antiballistic articles comprising suchnumber of sheets have improved antiballistic properties when compare tosheets comprising the fibrous tapes known in the art.

In a preferred embodiment, the antiballistic article has an arealdensity between 0.25 Kg/m² and 250 Kg/m², preferably between 0.5 Kg/m²and 100 Kg/m², more preferably between 1 Kg/m² and 75 kg/m² and mostpreferably between 2 Kg/m² and 50 Kg/m².

In a yet preferred embodiment, the antiballistic article of theinvention is a panel. By panel is understood herein that the individualsheets have been compressed, optionally under elevated temperature toform a single monolithic structure. Preferably, the panel of theinvention is compressed at a temperature of below the T_(m) of thepolymeric fibers, more preferably at a temperature of between said T_(m)and T_(m)−100 K and with a pressure of at least 100 bars, morepreferably at least 150 bars, to obtain a panel.

It was observed that the panels comprising the tapes according to theinvention have increased structural homogeneity and hence provide apanel with less fluctuation in the antiballistic properties compared topanels comprising fibrous tapes known from the prior art. The inventorsobserved that tapes with splits may lead to local fiber and/or tapedisplacements by portions of adjacent tapes being squeezed undermoulding conditions into said splits of the intermediate tape. Suchmigration of portions of tape result in reduced structural homogeneityof the antiballistic panel. Structural inhomogeneity can for example beobserved by microscopy of a cross-section of a compacted panel wherebythe cross-section is perpendicular to the common direction ofunidirectional aligned fibrous tape. In such cross-section (FIG. 1 andFIG. 2) unidirectional aligned fibrous tapes may be observed asdistinct, substantially parallel bands (1). A displacement due to asplit tape of a monolayer (2) may appear as a location in saidcross-section where the tapes of the 2 adjacent monolayers (3 and 4)originally separated by the tape of intermediate monolayer (2), contacteach other (5). It is the purpose of the present invention to provide apanel with an increased structural homogeneity, i.e. with a reducednumber of such tape contacts. The invention thus also relates to anantiballistic article comprising a number of contacts between 2 tapesseparated by at least one intermediate tape, alternatively calledseparating tape, wherein the number of contacts is less than 20 per unitof width of 1 meter of intermediate tape, while a contact corresponds toa tape-to-tape interaction of two tapes separated by an intermediatetape, occurring through a split of the intermediate tape. Such contactscan easily be counted by the skilled person when analysing across-section of said panel as depicted in FIG. 1 and FIG. 2.

The invention further relates to an armor comprising the panel of theinvention. Examples of armors include but are not limited to helmets,breast plates, vehicle hulls and vehicle doors.

The present invention further relates to a product for automotiveapplications such as car parts, etc.; marine applications such as ships,boats, panels, etc.; aerospace applications such as planes, helicopters,panels, etc.; defense/life-protection applications such as ballisticprotection, body armor, ballistic vests, shields, ballistic helmets,ballistic vehicle protection, etc.; architectural applications such aswindows, doors, walls, pseudowalls, cargo doors, cargo walls, radomes,shields, etc. wherein said product contains the tapes, the sheets or thepanel of the invention.

FIGS. 1 and 2 show a light microscopy picture at 2 different scalesshowing a portion of a cross-section (1) through a panel comprisingmonolayers the fibrous tapes according to the invention. The monolayersof which the fibers run substantially in parallel to the cross sectionare of a lighter shade (3 and 4) than the monolayers of which the fibersrun substantially perpendicular to the cross-section (2). The position(5) in the figures indicates a split of the tape of monolayer (2), whichsplit has been filled with the tapes of the respective adjacentmonolayers (3) and (4).

The invention will be further explained with the help of the followingexamples without however being limited thereto.

EXPERIMENTAL Methods of Measuring

-   -   Areal density (AD) of a panel or sheet was determined by        measuring the weight of a sample of preferably 0.4 m×0.4 m with        an error of 0.1 g. The areal density of a tape was determined by        measuring the weight of a sample of preferably 1.0 m×0.03 m with        an error of 0.1 g.    -   Intrinsic Viscosity (IV) is determined according to        ASTM-D1601/2004 at 135° C. in decalin, the dissolution time        being 16 hours, with DBPC as anti-oxidant in an amount of 2 g/l        solution, by extrapolating the viscosity as measured at        different concentrations to zero concentration. There are        several empirical relations between IV and Mw, but such relation        is highly dependent on molar mass distribution. Based on the        equation M_(w)=5.37*10⁴ [IV]^(1.37) (see EP 0504954 A1) an IV of        4.5 dl/g would be equivalent to a M_(w) of about 422 kg/mol.    -   Side chains in a polyethylene or UHMWPE sample is determined by        FTIR on a 2 mm thick compression molded film by quantifying the        absorption at 1375 cm⁻¹ using a calibration curve based on NMR        measurements (as in e.g. EP 0 269 151)    -   Tensile properties, i.e. strength and modulus, of fibers were        determined on multifilament yarns as specified in ASTM D885M,        using a nominal gauge length of the fibre of 500 mm, a crosshead        speed of 50%/min and Instron 2714 clamps, of type Fibre Grip        D5618C. For calculation of the strength, the tensile forces        measured are divided by the titre, as determined by weighing 10        meter of fibre; values in GPa for are calculated assuming the        natural density of the polymer (ρ), e.g. for UHMWPE is 0.97        g/cm³.    -   The tensile properties of tapes and films are determined        accordingly on tapes of a width of 2 mm twisted 40 turns per        meter.    -   The transversal strength of tapes is measured on a Zwick Z005        tensile tester with a 1 kN force cell and manual G13T and G13B        tape clamps. The test samples are prepared by manually cutting        the tapes to 250 mm strips. During preparation of the test        samples care should be paid to avoid unintentional partial        splitting of the tape. Samples which prove impossible to be cut        into coherent 250 mm strips are assigned a transverse strength        of 0 MPa. The clamping length of the samples is 60 mm and        distance between clamps is 20 mm. Pre-load is 0.1 N, and test        occurs at a speed of 50 mm/min. The maximum force determines the        transversal strength. The strength in MPa is calculated by        dividing that maximum force in Newton by the width and the        thickness of the sample in mm. Thus a breaking stress in N/mm²        is achieved, being identical to breaking stress in MPa. The        average of 5 samples is reported.    -   The melting temperature (T_(m)) of a filament is determined by        DSC on a power-compensation Perkin Elmer DSC-7 instrument which        is calibrated with indium and tin with a heating rate of 10        K/min on a 5 mg sample. For calibration (two point temperature        calibration) of the DSC-7 instrument about 5 mg of indium and        about 5 mg of tin are used, both weighed in at least two decimal        places. Indium is used for both temperature and heat flow        calibration; tin is used for temperature calibration only.    -   Ballistic performance was measured by subjecting the panels to        shooting tests performed with the further indicated ammunition.        The first shot was fired at a projectile speed (V₅₀) at which it        is anticipated that 50% of the shots would be stopped. The        actual bullet speed was measured at a short distance before        impact. If a stop was obtained, the next shot was fired at an        anticipated speed being 10% higher than the previous speed. If a        perforation occurred, the next shot was fired at an anticipated        speed 10% lower than the previous speed. The result for the        experimentally obtained V50 value was the average of the two        highest stops and the two lowest perforations. The kinetic        energy of the bullet at V₅₀ (E_(kin)=½·m·V₅₀ ²) wherein m is the        mass of the projectile, was divided by the areal density of the        armor to obtain a so-called E_(abs) value. E_(abs) reflects the        stopping power of the armor relative to its weight/thickness        thereof. The higher the E_(abs) the better the armor is.    -   The speed of the projectile was measured with a pair of Drello        Infrared (IR) light screen Type LS19i3 positioned perpendicular        on the path of the projectile. At the instant when a projectile        passes through the first light screen a first electric pulse        will be produced due to the disturbance of the IR beam. A second        electric pulse will be produced when the projectile passes        through the second light screen. Recording the moments in time        when the first and the second electric pulses occur, and knowing        the distance between the light screed the speed of the        projectile can be immediately determined.    -   Tape-to-tape contacts in a panel are determined by light        microscopy of polished cross-sections of a panel. The number of        contacts is counted on a cross-section of at least 1 by 5 mm².        The total width of cross-sectioned tape (in m) present in said        cross-section of 1 by 5 mm² is calculated multiplying 0.005 m        cross-section length by the number of visible cross-sectioned        tape, i.e. by dividing the height by the tape thickness.

General Experimental Setup

8 multifilament UHMWPE yarns were spread to form a homogeneous layer offilaments with a total thickness of about 50 micron and a width ofapproximately 3 cm. The filament layer was run through a set of twocounter rotating calender rolls of a diameter of 40 cm and a width of 4cm each. The temperature controlled rolls were set at a speed of 530cm/min and applied a pressure of 1750 N/mm to the filament layer. Afurther roller stand placed after the calender rolls applied a tensileforce of about 80 N to the tape exiting the nip of the calender rollsand optionally perform a post draw to the formed fibrous tape beforebeing air cooled to a temperature below 80° C. and wound on a bobbin.

Comparative Experiment A

A multifilament UHMWPE yarn with a tenacity of 3.1 N/tex and aparaffinic solvent level of about 50 ppm was subjected to above generalexperimental setup. The calender rolls were heated to 161° C. Theobtained fibrous tape A had an average thickness of 47.2 micrometer, awidth of 28 mm, a titre of 957 dtex and a tenacity of 3.01 N/tex. Thetransversal strength of the tape was 0.39 MPa.

Comparative Experiment B

Comparative Experiment A was repeated with the difference that a forcedair convection oven at a temperature of 143° C. in between two rollerstands applying an tensile force to the filaments was placed before thecalender rolls. The tensile force was adjusted to apply a draw rate of1.12 to the filament layer before entering the calender rolls set at atemperature of 160° C. The obtained fibrous tape B had an averagethickness of 47 micrometer, a width of 29 mm, a titre of 968 dtex and atenacity of 2.83 N/tex. The transversal strength of the tape was 0.41MPa.

Example 1

Comparative Experiment B was repeated with the addition that the drawnfilament layer was passed over a heated surface of 157° C., the contactpath with the heated surface having a length of about 3 cm beforeentering the calendering rolls set at a temperature of 139° C. Theobtained fibrous tape 1 had an average thickness of 39.6 micrometer, awidth of 30 mm, a titre of 751 dtex and a tenacity of 3.19 N/tex. Thetransversal strength of the tape was 0.60 MPa representing about a 50%improvement over the transversal strength of tape B.

1. A fibrous tape made from fibers comprising highly oriented polymer,the tape having a tenacity of at least 1.2 N/tex and an areal density ofbetween 5 and 250 g/m², wherein the tape has a transversal strength ofat least 0.5 MPa.
 2. The tape of claim 1, wherein the polymer is apolyolefin, preferably a polyethylene and more preferably a high orultrahigh molecular weight polyethylene (UHMWPE).
 3. The tape of claim 1wherein the tape has a cross-sectional aspect ratio thickness to widthof at most 1:50, preferably at most 1:100, more preferably at most1:500.
 4. The tape of claim 1 wherein the tape has a tenacity of atleast 1.5 N/tex, preferably at least 2.0 N/tex, more preferably at least2.5 N/tex, even more preferably 3.0 N/tex and most preferably 3.5 N/tex.5. The tape of claim 1 wherein the transversal strength is at least 0.6MPa, more preferably at least 0.7 MPa, even more preferably at least 0.8MPa, and most preferably at least 0.9 MPa.
 6. The tape of claim 1wherein the fibers comprising the polymer comprise at least 10 ppm of asolvent for the polymer.
 7. A sheet comprising at least two monolayerscomprising fibrous tape or at least one layer of woven fibrous tapes,wherein the fibrous tapes are selected from claim
 1. 8. The sheetaccording to claim 7 wherein the direction of the fibrous tape in amonolayer is at an angle α to the direction of a fibrous tape in anadjacent monolayer or wherein the layer of woven fibrous tapes comprisesweft and warp woven tapes and the direction of orientation of the weftand the warp woven tapes in the layer of woven fibrous tapes are at anangle β and wherein α or β are between 20 and 90°, more preferablybetween 45 and 90°, most preferably between 75 and 90°.
 9. Anantiballistic article comprising at least 2, preferably at least 4, morepreferably at least 8 sheets according to claim
 7. 10. The antiballisticarticle of claim 9 having an areal density between 0.25 Kg/m² and 250Kg/m², preferably between 0.5 Kg/m² and 100 Kg/m², more preferablybetween 1 Kg/m² and 75 kg/m² and most preferably between 2 Kg/m² and 50kg/m².
 11. The antiballistic article of claim 9, comprising a number ofcontacts between two tapes separated by an intermediate tape, whereinthe number of contacts is less than 20 per unit of width of 1 meter ofthe intermediate tape, while a contact corresponds to a tape-to-tapeinteraction of the two tapes separated by an intermediate tape,occurring through a split of the intermediate tape.
 12. A process forthe preparation of the fibrous tape of claim 1, the process comprising:(a) providing fibers comprising a highly oriented polymer, said fibershaving a tenacity of at least 1.2 N/tex (b) forming a layer comprisingthe fibers; (c) applying a longitudinal tensile force to the fibers inthe layer, (d) stretching the fiber layer at a draw ratio of at least1.01 to form a stretched layer; (e) providing the stretched layer at aprocessing temperature T_(p) to compression means; (f) compressing thestretched layer of fibers by subjecting the layer to a compression bythe compression means having a temperature T_(c) to form a fibrous tape;(g) optionally stretching the fibrous tape by a draw rate of at most 1.1and, (h) cooling the fibrous tape to a temperature of at most 80° C.under a tension sufficient to prevent loss of mechanical properties;wherein T_(m) is the melting temperature of the polymer, whereinT_(m)>T_(p)≧T_(m)−30 K, and wherein T_(c)≦T_(p)−3 K.
 13. The processaccording to claim 11 wherein T_(m)>T_(p)≧T_(m)−15 K, and whereinT_(c)≦T_(p)−15 K.
 14. The process according to claim 11 wherein thepolymer is UHMWPE, preferably the UHMWPE has an intrinsic viscosity ofbetween 5 dL/g to 40 dL/g, more preferably between 8 and 30 dL/g. 15.The process of claim 11 wherein the filaments are stretched in betweenthe steps (a) and (f) to a draw ratio from 1.02 to 3.0, preferably 1.03to 2.0.